Polyamide resins, which are excellent in mechanical strength and heat resistance, have been hitherto used in the fields of automobile parts, mechanical parts and electrical and electronic parts. In the field of electrical and electronic parts especially, the required level of flame retardancy becomes higher and higher. Therefore, polyamide resins are required to exhibit higher level of flame retardancy than their inherent self-extinguishing property. A number of investigations have thus far been made to obtain a high level of flame retardancy, to be more specific, to develop a material which is rated V-0 level under the UL94 standard from Underwriters Laboratories. In particular, even in the case of compositions compounded with an inorganic filler, there is a strong demand for development in which a non-halogen type flame retardant is used according to the trends of the time. In such applications, in particular, in applications for various kinds of breakers that are generally connected to electric wirings, such as a safety breaker, an ampere breaker, a ground-fault interrupter, a circuit breaker, MCCB (molded case circuit breaker), NFB (no-fuse breaker), a motor protecting breaker (motor breaker), a slim breaker (one-touch breaker or one-touch breaking breaker), etc., requirements for glow wire characteristics and tracking resistance characteristics which are typical standard of European IEC become higher and higher in relation to safety assurance of electric wirings.
A polyamide resin composition which contains a melamine cyanurate and/or a derivative thereof and an inorganic filler and which is alleged to have good mechanical properties, thermal characteristics and flame retardancy is proposed (Patent Document 1). The composition, however, has flame retardancy of V-2 level under the UL94 standard. No resin compositions having higher flame retardancy, namely of V-1 level or higher are proposed.
A flame proof thermoplastic molding material which contains a melamine cyanurate and, compounded therein, a glass fiber having a determined distribution of the length of the glass fiber, in particular a short glass fiber and which is alleged to have excellent mechanical property and flame proofing property is proposed (Patent Document 2). The most of the specifically proposed resin compositions, however, have flame retardancy of V-2 level (at a thickness of 1/16 inch) under the UL94 standard. There is no mention at all of tracking characteristics, extrudability, moldability or appearance of molded articles.
A proposal is made for a flame retardant containing a first component (component comprising a phosphinic acid salt having a specific structure and/or a diphosphinic acid salt having a specific structure and/or a polymer of thereof) and a second component (a melamine condensate and/or a reaction product of a melamine with a phosphoric acid and/or a reaction product of a melamine condensate with a phosphoric acid and/or a mixture thereof) and for flame proofing of a thermoplastic polymer (Patent Document 3). More concretely, a polyamide resin composition which comprises a reinforced polyamide resin comprising 30% by weight of a glass fiber and, compounded therein, a flame retardant composed of a phosphinic acid salt (first component) and a melamine polyphosphate (second component) is proposed The polyamide resin composition has flame retardancy of V-0 level (at a thickness of 1/16 inch) under the UL94 standard. However, there is no description at all concerning other properties such as moldability, mechanical properties and electrical properties of the polyamide resin composition.
A flame retardant polyamide resin composition compounded with the above-described flame retardant (containing the first and second components each in an amount of 1 to 30% by weight) and 5 to 40% by weight of an inorganic filler (glass fiber, wollastonite, talc, calcined kaolin, mica, etc.) is proposed (Patent Document 4). More concretely, there is proposed a polyamide resin composition comprising 20% by weight or 30% by weight of a glass fiber which is capable of exhibiting flame retardancy of V-0 level (at a thickness of 1/32 inch) under the UL94 standard and tracking property in terms of CTI of 550 to 600 V. It is also proposed that a flame retardant aid such as magnesium hydroxide, aluminum hydroxide, zinc sulfide, iron oxide, boron oxide and zinc borate may be compounded in the composition. However, there is no description concerning the compounding amount and the effect obtained by the addition of such a flame retardant aid. Further, there is shown no concrete proposal as to a resin composition containing the flame retardant aid.
It is pointed out that the above flame retardant is problematic with respect to foaming of strands and generation of a large amount of gases during extrusion, occurrence of mold staining (mold deposits) during a molding process, and reduction of fluidity and mold releasability (Patent Document 4). A re-test performed by the present inventors has revealed that the proposed resin composition, in which the above-mentioned flame retardant (comprising the first and second components each in an amount of 1 to 30% by weight) and 5 to 40% by weight of an inorganic filler are compounded, cannot solve these problems and that a further technical improvement is required. It has been also revealed that when a molded article obtained using the proposed resin composition is exposed to a high temperature and high humidity atmosphere, the flame retardant bleeds out from the surface thereof to cause problems such as surface roughening of the molded article and deterioration of the flame retardancy.
A combined flame retardant and stabilizing agent for thermoplastic polymers is proposed which comprises 20 to 80% by weight of (A) a phosphinic acid salt and/or a diphosphinic acid salt and/or a polymer thereof (corresponding to the first component disclosed in Patent Document 3), 10 to 75% by weight of (C) a phosphorus/nitrogen flame retardant (corresponding to the second component disclosed in Patent Document 3), and further 10 to 75% by weight of (B) a nitrogen-containing synergist (Patent Document 5). It is alleged that a thermoplastic resin composition obtained by compounding the above flame retardant and stabilizing agent into a thermoplastic resin exhibits good glow wire stability and that when a zinc compound (such as zinc borate, zinc oxide or zinc stannate) is preferably additionally compounded therein, stability during processing is also improved. It has been found as a result of a study made by the present inventors, however, that such a resin composition still fails to solver in a sufficient manner, problems of deterioration of the appearance of molded articles, due to generation of gases during a molding process and bleeding-out of the flame retardant, staining of the molds and the like.
Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 54-16565 (1979)
Patent Document 2: Japanese Translation of PCT International Application No. 11-513059 (1999)
Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2001-72978
Patent Document 4: Japanese Patent Application Laid-Open (KOKAI) No. 2004-292755
Patent Document 5: Japanese Patent Application Laid-Open (KOKAI) No. 2005-240035